Ed Casey has informed the services group that he wants to return to the US and has accepted a role with another company, which is not a competitor. He will depart Serco on 31 December.
Serco chief executive Rupert Soames said: "Whilst we are very sad that Ed is leaving, we completely understand that after four years of weekly commuting across the Atlantic, he would like to have a job closer to home.
"I would like to place on record our sincere gratitude for all he has done for Serco over the last 12 years, and in particular helping to lead the company through the critical early years of the transformation of our business."
Serco provides an assortment of services, including prison management, transportation, call centres and school services.
Over the past few years, the company has been trying to put difficulties like the electronic tagging scandal of 2013 in the past. In its last set of results, the group posted figures in line with expectations, with analysts and investors alike seemingly pleased with the "reassuringly dull" numbers.